1. Field of Invention.
This invention relates to a subminiature current transformer which is capable of withstanding high voltages and which includes a frequency compensating device.
2. Description of the Prior Art.
A current transformer is used when it is desired to introduce electrical signals from a commercial power line into an analog IC (integrated circuit). Since such a current transformer is normally used together with the analog IC, both are usually attached to the same circuit board. It is desirable to have the combined structure as small as possible.
The Japanese Industrial Standard(JIS), for example, requires that such current transformers have a capacity to withstand high voltages on the primary side, for example, of 3000 volts AC for one minute, and 4500 impluse volts for 1.times.40 .mu.sec. Other industrial standards, such as the ANSI, similarly requires such current transformers to have the capacity to withstand high voltages.
As shown in FIG. 1, the conventional current transformer is configured to have its primary coil N1 and secondary coil N2 wound about a single bobbin B, in an overlapped manner. To handle the high voltages required by such standards, an insulating layer S, together with a shield, are provided between the primary and secondary coils. However, disadvantageously, in such current transformers having the insulating sheet, it is impossible to provide sufficient creepage distance between the primary and secondary coils. Thus, the more the current transformer is reduced in size, so as to fit the IC input, the more difficult it becomes to satisfy the high withstanding voltage standards. Accordingly, in the art, the conventional current transformers are inevitably large sized, that is not within the subminiature size range.
If the conventional current transformer is reduced in size to the subminiature range, the frequency characteristic, as well as the capacity to withstand high voltages, are degraded. This is because core loss increases with increase in degree of miniaturization, and there results in a characteristic having a "corner" in the frequency range which is higher than the commercial frequency.
Thus, there is a need in the art for a subminiature current transformer which can meet the industrial standards regarding capacity to withstand high voltages e.g. at the primary windings, and which has the other advantageous characteristics of being relatively error free.